managed to revive my old battery

[foama]

Usually on the OEM style Hitachi battery, the fluid does not need to be topped up during its lfe. Fluid level will become a bit lower over time and acidity increases accordingly. It can be topped up to prolong battery life, because higher acidity damages the battery. If fluid drops more on somebody's car than usual, that has a cause. Perhaps the charging voltage is a bit on the high side.

The positive battery plates will develop an increasing layer of phosphate alloy over time. That reduces the charge and amperage a battery can produce. Some battery chargers have a "regenerate" mode, which will charge the battery at around 15.5 to 16.0Volts, causing the fluid to discharge water as a (hghly combustable!) gas mixture. It should be left in a ventilated place in regenerate mode for about 12 hours. That gas mixture is oxgen plus hydrogen being depleted water taken out of the fluid by electrolysis. During regenerate mode it forms bubbles on the positive plates that loosen and remove some of the phosphate build-up formed over time, and thus the battery regains some of its former performance.
After about five years use, a perfectly working battery will have developed enough phosphate on the positive plates to have lost some of its performance capability. This is where a "regenerate" charge will make an improvement to prolonging its life. After regenerating, fluid level should be checked, to ensure missing water has not increased its fluid acidty, which would shorten the batteries life. Do not overfill.
A regenerate-mode charge once every year or so pays off.

If your battery is not performing very well, it is worth a try topping it up wth distilled water, and to use a "regenerate mode" charge at around 16 volts for about 12 hours. It then should have regained a good part of its former capacity.

[Wallythacker]

Usually on the OEM style Hitachi battery, the fluid does not need to be topped up during its lfe. Fluid level will become a bit lower over time and acidity increases accordingly. It can be topped up to prolong battery life, because higher acidity damages the battery. If fluid drops more on somebody's car than usual, that has a cause. Perhaps the charging voltage is a bit on the high side.

The positive battery plates will develop an increasing layer of phosphate alloy over time. That reduces the charge and amperage a battery can produce. Some battery chargers have a "regenerate" mode, which will charge the battery at around 15.5 to 16.0Volts, causing the fluid to discharge water as a (hghly combustable!) gas mixture. It should be left in a ventilated place in regenerate mode for about 12 hours. That gas mixture is oxgen plus hydrogen being depleted water taken out of the fluid by electrolysis. During regenerate mode it forms bubbles on the positive plates that loosen and remove some of the phosphate build-up formed over time, and thus the battery regains some of its former performance.
After about five years use, a perfectly working battery will have developed enough phosphate on the positive plates to have lost some of its performance capability. This is where a "regenerate" charge will make an improvement to prolonging its life. After regenerating, fluid level should be checked, to ensure missing water has not increased its fluid acidty, which would shorten the batteries life. Do not overfill.
A regenerate-mode charge once every year or so pays off.

If your battery is not performing very well, it is worth a try topping it up wth distilled water, and to use a "regenerate mode" charge at around 16 volts for about 12 hours. It then should have regained a good part of its former capacity.

I will mention some of the devices you can use to restore a battery will send high frequency AC pulses into the battery. The idea is the pulses will crack and force the sulphates on the plates to dissipate. I have mixed luck using these, some batteries respond well to this tech and others, well, nothing changes. I found smaller batteries, like powersports and lawn tractors respond OK, large deep cycle RV and Marine batteries, not so well.

The thing is, you don't know for sure the reason for a battery loosing capacity other than age the battery physically breaking down internally, which includes getting sulphated.

[Wallythacker]

One more thing.
Once, I goofed and poured hydrochloric acid into a battery and it sat there for a good hour before I clued in.

I drained that acid, flushed the battery with distilled water and added sulfuric acid, left it on the cheap charger overnight and the next day installed the battery and used it for about 6-7 years and never bothered to pull the battery when I scrapped or sold the car.

Maybe, just maybe the hydro acid attacked and completely destroyed the phosphate layer on the plates. I'm sure a chemist could chime in and tell us what happens when you mix acids like I did and the environment they are in at the time.